Homologous antisera prepared with homogeneous immunoglobulins (myeloma proteins) or normal immunoglobulins identify immunoglobulin IgCH allotypes and Fab idiotypes in the mouse. These antisera have been used in a variety of genetic studies involving tests for linkage, dominance and recombination. Specific anti-idiotype antisera have been particularly useful in studies of variable region site associated cross specific idiotypes on myeloma proteins that bind the same haptens. Normal antibodies made to the same hapten in some strains share these idiotypes; in others they are absent. The genes controlling these idiotypes were found to be linked to the Ig-CH region. By determining the frequency of recombination between IgVH and IgCH genes and between different VH genes, it may be possible to estimate the number of genes required for the diversity of antibody. In addition to studies of the structural genes for immunoglobulins it is also important to understand the mechanisms that control the reponse to particular antigens. The homogeneous immunoglobulins (myeloma proteins) are useful as immunogens to study the genetics of the immune response. The response to particular classes of immunoglobulins, e.g. IgA and IgGzb, have been found to be controlled by the H-2 linked immune response genes Ir-1a and Ir-1b.